1. Field of the Invention
This invention relates to a sensing circuit for vibration type of angular rate sensor (i.e., angular velocity sensor) provided with a malfunction detector. In particular, the present invention relates to a sensing circuit provided with a malfunction detector for detecting, with high accuracy, variation of driving impedance caused such as by attachment of foreign matter to a vibrator, the sensing circuit being applied, for example, to a vibration type of angular rate sensor used such as for vehicle control.
2. Description of Related Art
As a system for performing vehicle control by means of an angular rate sensor, stable control systems for vehicle, for example, are known in which vehicle sideslip is detected to optimally control a brake and torque of a vehicle to thereby maintain the vehicle in a normal condition. As such a system, four-wheel rudder angle control systems, for example, are also known in which a rudder angle of rear wheels or front wheels of a vehicle is controlled.
These types of systems typically detect malfunction conditions of a vehicle, such as sideslip, with a yaw rate signal, i.e. by means of an angular rate sensor. Malfunction of a yaw rate signal means a possible unstableness of the traveling characteristics of a vehicle, which may cause unexpected behavior of the vehicle.
One example of a technique for resolving the problem described above is disclosed in Published Japanese Unexamined Patent Application No. H11-051655. This technique includes a device for examining whether or not an angular rate sensor is in a normal operation. The device is so arranged to detect that a drive voltage corresponding to a driving force for a vibrator, has gone off a specified voltage, by means of a constant voltage circuit and a comparator, and to output self-diagnosis as a diagnostic detection signal.
Japanese Patent No. 2084567 discloses a technique in which the amplitude of a vibrator in a driving direction is detected, and amplitude control and 90-degree phase shift are effected in order to stabilize zero point/sensitivity of an angular rate signal, so that feedback control is effected as a drive signal.
A malfunction detection circuit, which is the combination of the techniques disclosed in the above publications, has been provided to a sensing circuit in an angular rate sensor. Specifically, a sensing circuit provided with a malfunction detection circuit, as shown in FIG. 4, has generally been employed.
As shown in FIG. 4, a sensing circuit in an angular rate sensor is so configured that it comprises a vibrator 30, an amplitude control circuit 40, and a malfunction detection circuit 50.
The vibrator 30 is provided with a driving sensor element and a pair of sensor elements for detecting yaw (not shown), and is so configured that, if yaw occurs when the driving sensor element is effecting driving vibration, the pair of detection sensor elements are vibrated by the Coriolis force. Since the vibrator 30 generates outputs corresponding to the respective vibrations of the pair of detection sensor elements, yaw can be detected based on the outputs. The vibrator 30 is so configured that it also generates an output corresponding to the driving vibration in order to detect that the driving sensor element is adequately effecting driving vibration.
The output from the vibrator 30 corresponding to the driving vibration, i.e. a vibration amplitude in a driving direction, is converted to a voltage in an i/v converter circuit (or C-V converter circuit) 41 and then passed to a rectification circuit 42 in order to obtain a DC voltage, which is equivalent to the vibration amplitude. Then, in a differential amplifier 44, an error voltage with reference to Vref1, which is generated by a first reference-voltage generation circuit 43, is detected as an error signal S61.
On the other hand, in order to permit the vibrator 30 to vibrate at a frequency of normal mode of vibration (resonance frequency) of the vibrator 30, a signal corresponding to the vibration amplitude, i.e. the output, of the i/v converter circuit 41 is passed to a 90-degree phase-shift circuit 46. The signal is then multiplied with the error signal S61, which is an output from the differential amplifier 44, in a multiplier 45 in an amplitude control circuit 40, for feeding back to the vibrator 30 as a driving voltage. As a result, the vibrator 30 can effect vibration at the resonance frequency, so that the amplitude is kept at a constant level.
If malfunction occurs, such as an attachment of foreign matter to the vibrator 30, to increase driving impedance, the driving amplitude of the vibrator 30 is decreased, which in turn decreases the output of the rectification circuit 42, making larger the difference between the rectifier output and the Vref1 generated by the first reference-voltage generation circuit 43. As a result, the error signal S61, which is an output of the differential amplifier 44, is magnified, and thus the driving voltage, which is an output of the multiplier 45, is increased. In this way, the driving amplitude is controlled so as to keep the driving amplitude of the vibrator 30 at a constant level.
In the vibrator 30 and the driving circuit 40 configured as described above, in order to detect malfunction of the vibrator 30 caused such as by the attachment of foreign matter, all that is required is to detect whether or not the error signal S61, which is an output of the differential amplifier 44 and is equivalent to the driving impedance, is within a specified range.
Thus, as shown in FIG. 4, an arrangement is made such that the error signal S61 is inputted to a window comparator 53 in a malfunction detection circuit 50, and that the error signal S61 is detected as to whether or not the voltage thereof is within a voltage ranging from Vref2 generated by a second reference-voltage generation circuit 51 to −Vref2 formed by an inverting circuit 52. If the voltage of the error signal S61 does not fall in this voltage range, a diagnostic detection signal S62 is adapted to be outputted.
The driving impedance of the vibrator 30, however, is unavoidably varied depending such as on the dimensional accuracy of the vibrator 30 and due to temperature variation and aging variation. Accordingly, circumstances have been such that, as a criterion for detecting malfunction of the vibrator 30 for the error signal which corresponds to the driving impedance of the vibrator 30, such values as described above in consideration of errors had to be set. Because of this, setting of values which are essentially required for malfunction detection have not been enabled, and thus no high-accuracy malfunction detection has been enabled.
Moreover, with the recent trend of downsizing of vibrators, vibrators are being replaced by those of semiconductor type which carry out detection in terms of capacitance. This, in turn, has come to give impact on zero point and sensitivity of an angular rate sensor with even the attachment of very small foreign matter which conventionally caused no problem. Thus, there is an increasing need for a malfunction detector having high accuracy and high reliability.